1. Field of the Invention
The present invention relates to a method for producing a magneto-optical recording medium made from a plurality of magnetic layers connected to one another by exchange interaction between which layers a boundary magnetic domain wall can exist around a room temperature.
2. Description of the Prior Art
In the field of magneto-optical recording and reproduction, a request for a higher transfer rate has been raised. In order to answer this request, various techniques have been developed and are being implemented in practice. Among them, there can be named a technique of photo-intensity modulation direct overwriting.
When recording data on a magneto-optical recording medium with a photo-intensity modulation method, normally, a weak DC magnetic field is applied to the magneto-optical recording medium and a laser beam is radiated while modulating the beam depending on the presence/absence of a signal. Consequently, when re-recording a data on a already recorded area by using a conventional magneto-optical recording/reproduction apparatus of the photo-intensity modulation method, it is necessary to erase the recorded area prior to the re-recording. That is, it is impossible to directly overwrite on the already recorded area.
Thus, most of the magneto-optical recording/reproduction apparatuses which have been implemented in practice require an erase operation prior to recording a new data. This means that the magneto-optical recording medium should rotate by at least two turns for recording a signal, which has prevented to provide a high transfer rate.
In order to solve such a problem, there has been designed the photo-intensity modulation direct overwriting technique which employs the photo-intensity modulation method and enables direct overwriting.
For example, Japanese Patent Laid-Open Sho 62-175948 discloses a photo-intensity modulation direct overwriting method which employs a magneto-optical recording medium made from two magnetic layers layered having different magnetic characteristics and layered so as to be connected to each other by exchange interaction; and a magneto-optical recording and reproduction apparatus including: an optical system capable of radiating a beam controlled to two values, i.e., a high level and a low level, during a recording; a recording magnetic field generating apparatus which is used in an ordinary magneto-optical recording and reproducing apparatus; and a so-called initialization magnet, i.e., an external magnetic field generating apparatus capable of inverting only one of the two layers at a room temperature.
However, in order to realize a photo-intensity modulation direct overwriting by using this technique, the initialization magnet should an initialization magnetic field in the order of several kOe, which is a problem in designing the magneto-optical recording and reproduction apparatus.
In order to cope with this, reduction of the initialization magnetic field has been tried such as in a method disclosed in Japanese Patent Laid-Open Hei 1-185853. Such efforts have reduced the initialization magnetic field in the order of 2 kOe, which is still a significantly great value if compared to the recording magnetic field in the order of 300 Oe. This initialization magnetic field is still a problem and this method is not yet implemented in practical use.
On the other hand, separately from the examination for reduction of the initialization magnetic field, a research has been made to enable the photo-intensity modulation direct overwriting without using any initialization magnetic field. As a result, there has been designed a magneto-optical recording medium disclosed in Japanese Patent Sho 63-268103 and further in Japanese Patent Hei 3-219449.
This magneto-optical recording medium basically consists of the magneto-optical recording medium disclosed in Japanese Patent Laid-Open Sho 62-175948 with addition of a magnetic layer which serves as an initialization magnet by the exchange interaction force. This magnetic layer is prepared in such a manner that its magnetization will not change during a recording and is called an initialization layer. That is, this magneto-optical recording medium enables photo-intensity modulation direct overwriting by providing the initialization layer whose magnetization direction is not changed during a recording.
The magneto-optical recording medium capable of carrying out the photo-intensity modulation direct overwriting by means of the initialization layer includes at least: a magnetic layer which is magnetized according to a recording signal; a magnetic layer whose magnetization direction is temporarily changed according to a recording signal during a recording; a magnetic layer which is temporarily demagnetized during a recording; and a initialization layer whose magnetization direction is not changed during a recording;
Here, the magnetic layer which is magnetized according to a recording signal is called a memory layer which is a layer for retaining recording magnetic domain. The magnetic layer whose magnetization direction is temporarily changed according to a recording signal during a recording is called a recording layer which is magnetically inverted according to a recording signal during a recording and initialized to a particular direction upon completion of the recording. The magnetic layer which is temporarily demagnetized is called a switch layer which is for controlling a magnetic connection state between the recording layer and the initialization layer.
In such a magneto-optical recording medium, magnetization of the memory layer is inverted by utilizing a temperature dependency of the energy, i.e., which is greater: the boundary magnetic wall energy between the memory layer and the recording layer or the coercive force of the memory layer.
In the aforementioned magneto-optical recording medium capable of the photo-intensity modulation direct overwriting by means of the initialization layer, the magnetization direction of the memory layer and that of the recording layer are parallel or anti-parallel to each other according to a recording signal. Consequently, it is necessary that a boundary magnetic wall exist with a stability between the memory layer and the recording layer around a room temperature.
That is, in a magneto-optical recording medium capable of the photo-intensity modulation direction overwriting, it is necessary that the recording domain formed in the memory layer be retained with a sufficient stability under the existence of the boundary magnetic wall between the memory layer and the recording layer. However, it is difficult to retain such a boundary magnetic wall with a stability. Consequently, in the magneto-optical recording medium capable of carrying out the photo-intensity modulation direction overwriting, it is difficult to assure a reproduction stability compared to the conventional magneto-optical recording medium having no such boundary magnetic wall.
Moreover, in the magneto-optical recording medium capable of carrying out the photo-intensity modulation direct overwriting, the demagnetization process utilizes the temperature dependency of the relationship between the effective magnetic field by the boundary magnetic wall and the coercive force of the memory layer. However, this temperature dependency is susceptible to a thermal damage by repeated recording processes. Consequently, in the magneto-optical recording medium capable of carrying out the photo-intensity modulation direct overwriting, it is difficult to suppress the demagnetization sensitivity change after the thermal damage caused by repeated recording processes, compared to the conventional magneto-optical recording medium in which the magnetization sensitivity is almost determined by the Curie temperature.
As has thus far been described, the magneto-optical recording medium capable of carrying out the photo-intensity modulation direct overwriting has a problem of stability of the boundary magnetic wall formed between the memory layer and the recording layer, and there is an object to achieve a stability of the boundary magnetic wall.
Such an object applies not only to the magneto-optical recording medium capable of Carrying out the photo-intensity modulation direct overwriting, but also to a magneto-optical recording medium in general having a plurality of magnetic layers which are magnetically connected to one another by the exchange interaction effect so that a boundary magnetic wall can be exist between the magnetic layers around a room temperature.